epicycle - I was faced with your EXACT situation last month... we have some steep mountains out here and I had a standard force crank with 12-26 on the back.
I found a compact Red crank for about $200 on fleabay and it came with mid compact chainrings as well as the original compact chainrings (so mids are 52/36 and compact are usually 50/34)
I swapped out the cassette for a Force 11-28 and it's been awesome for climbing. With a 52 up front, I really haven't lost anything (actually, by chainging to an 11 cog from a 12, I picked up a lot of gear inches on the top end and I can hit 40 mph while pedaling slightly under 110 rpm.
I only use the 36 chainring when the road gets steeper than 4% and I don't even use the 28 cog until the road gets steeper than 8% or I'm really tired (and then it's a godsend).
Based on the way I pedal, and the actual cogs included, I think I'd probably be better off MOST of the time with a true compact and a 12-28 cassette. If you look at a chart with the gears & cadence vs speed you can figure out where you'll spend most of your time and I think you benefit from having close ratios at those speeds.
The 11/28 includes: 11,12,13,14,15,17,19,22,25,28
The 12/28 includes: 12,13,14,15,16,17,19,22,25,28
So you can see you lose a 16 tooth cog and gain an 11.
If you're going 20 mph at 85 rpm, you're in 52x17 and there's a two tooth gap on either side of your chosen gear.
If you're going 20 mph at 85 with a compact and 12-28, you're in the 16 cog, and you have 1 tooth jumps from 12 all the way to 17.
The scenarios get worse as you slow down because there are larger jumps from 19 to 28 on either cassette.
So depending on your normal cruise speed (for me, 18-22 is a respectable top end on flat ground with no wind) you may find that the above makes sense. Obviously, faster riders will want a larger big chainring and for people that NEED to bomb down hills, well, 53 is awesome.
Something to consider when changing out cranks - people are dumping their Red cranks because the new 2012 red is out, and shiny. If you can find a lightly used set, it's a great deal. If you buy a crank with chewed up chainrings, you'll be buying new chainrings soon enough and a set of those is $140 or so, so not much of a good deal.